[GHC] #7069: precision/rounding bug with floating point numbers on 32-bit-platforms

[GHC]

#7069: precision/rounding bug with floating point numbers on 32-bit-platforms
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  Reporter:  shahn                        |          Owner:  simonmar                          
      Type:  bug                          |         Status:  closed                            
  Priority:  normal                       |      Milestone:  7.6.1                             
 Component:  Compiler                     |        Version:  7.4.2                             
Resolution:  wontfix                      |       Keywords:  Float Double 32 precision rounding
        Os:  Unknown/Multiple             |   Architecture:  x86                               
   Failure:  Incorrect result at runtime  |     Difficulty:  Unknown                           
  Testcase:                               |      Blockedby:                                    
  Blocking:                               |        Related:                                    
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Comment(by maeder):

 Maybe it is enough that (true) floating point arithmetic yields non-
 deterministic results.
 The problem here regards equality, which need not be the same as "a - b ==
 0". So if we implement

 {{{
 a == b = show a == show b
 }}}

 (or more efficiently using rounding)

 then this better reflects the data model, i.e. for set instances (and the
 expected behavior for this ticket).

 Yet, almost all number laws are broken, i.e.

 {{{
 a == b ==> a - b == 0
 }}}

 But maybe this is more acceptable (with a broken FPU).

 The difference between the "equal" values 0.65999289 and 0.65999289 (of
 the given example) happens to be 5.551115123125783e-17 (so is clearly
 visible as Double).

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7069#comment:7>
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